In many instances, it is desirable to detect a fracture or crack in a workpiece such that the workpiece may repaired before the fracture enlarges and creates additional damage to the workpiece. For example, the outer surfaces or skin of most modern aircraft are comprised of a metal and, more particularly, are comprised of aluminum. Although modern aircraft are constructed according to rigid specifications, the conditions under which aircraft operate and the forces on an aircraft during service may cause the outer surface of the aircraft to fracture. These fractures typically occur in areas which are loaded with the largest forces during service, such as the bulkhead and engine mounts, as well as structurally weakened areas. Such structurally weakened areas include joints at which adjacent panels have been joined, such as by a row of rivets.
Fractured workpieces may, in many instances, be repaired. The repair of a fractured workpiece is particularly effective if the fracture in the workpiece is detected while the fracture is relatively small and before the workpiece suffers additional structural damage. For example, a fracture along the bulkhead or engine mounts of an aircraft may be repaired without hindering the performance of the aircraft if the fracture is detected while it is relatively small.
Workpieces are typically inspected manually, such as by a visual inspection, to detect fractures. However, manual inspection and detection of fractures in a workpiece, particularly a relatively large workpiece such as an aircraft, is time consuming and expensive. Thus, fracture detecting systems have been developed which periodically inspect the surface of a workpiece to identify any fractures in the workpiece. For example, eddy current fracture detection methods utilize eddy current probes to detect fractures in a workpiece. In addition, ultrasonic fracture detection systems have been developed to detect fractures in a workpiece. Ultrasonic fracture detection systems can typically only detect fractures in the line of sight between the ultrasonic transmitter/receiver. Thus, fractures within workpieces of a complex shape may not be detected by such ultrasonic fracture detection systems. In addition, a workpiece is only subjected to eddy current or ultrasonic inspections periodically such that fractures which occur between the periodic inspections will not be detected until a subsequent inspection. Without repair, these undetected fractures will typically enlarge and cause additional structural damage to the workpiece prior to being detected during the subsequent inspection.
Accordingly, other types of fracture detecting systems which continually monitor a workpiece have been developed. Such fracture detecting systems include the remote optical crack sensing system disclosed in U.S. Pat. No. 4,636,638 (hereinafter the "'638 patent") which issued Jan. 13, 1987 to Shih L. Huang et al. The crack detecting system of the '638 patent includes an optical fiber mounted on stress risers upon the surface of a workpiece, such as an aileron of an aircraft. The stress risers are adhesively mounted on the workpiece and extend outwardly therefrom. The crack sensing system also includes a light source for introducing light at a first end of the optical fiber and a detector at a second end of the optical fiber, opposite the first end, for receiving the light transmitted therethrough.
According to the crack sensing system of the '638 patent, cracks in the surface of the workpiece alter the relative positions of the stress risers and break the optical fiber. The light transmitted through the optical fiber is attenuated by the break in the optical fiber. Thus, by identifying the attenuation of the transmitted light with the detector, a break in the optical fiber and a corresponding fracture in the workpiece may be detected.
The crack sensing system of the '638 patent extends above the surface of the workpiece due, in part, to the stress risers. Therefore, the crack sensing system of the '638 patent alters the profile of the workpiece. In many instances, however, it is not desirable to alter the profile of a workpiece by mounting thereon objects which extend outwardly from the surface of the workpiece. For example, it is not desirable to alter the profile of an aircraft, as defined by the outer surface of the aircraft, by mounting additional devices, such as a fracture sensing system, thereon since any alterations in the profile of the aircraft changes the performance characteristics of the aircraft.
Accordingly, optical fracture sensing systems have been developed which are mounted directly on the surface of a workpiece, such as the surface of an aircraft. One such optical fracture sensing system is described in a publication entitled An Optical-Fibre Fatigue Crack-Detection and Monitoring System by K. F. Hale which was published in the Smart Materials and Structures, Vol. 1, p. 156 (1992) (hereinafter the "Hale publication"). The optical fiber crack detection and monitoring system disclosed by the Hale publication includes optical fibers mounted directly on the surface of the workpiece. The optical fibers are mounted with a high-modulus adhesive such that fractures in a portion of the workpiece underlying the optical fiber will also crack the optical fiber. Once an optical fiber has cracked, light transmitted through the optical fiber will be attenuated and the fracture in the workpiece will be detected.
The high-modulus adhesive which bonds the optical fiber to the surface of the workpiece is generally an organic epoxy. Organic epoxies, however, have a different coefficient of thermal expansion than metal, such as aluminum. Accordingly, the organic epoxy may de-bond from a metallic workpiece as the workpiece expands and contracts during temperature fluctuations. Once the organic epoxy de-bonds from the workpiece, the optical fiber is no longer affixed to the workpiece and no longer reliably detects fractures in the workpiece.
In addition, metallic workpieces and, in particular, aluminium workpieces rapidly develop a surface oxide layer upon exposure to air. Organic epoxies, however, cannot readily bond to a workpiece covered with an oxide layer. Accordingly, the oxide layer must be removed from the surface of a workpiece, typically by chemically and mechanically cleaning and preparing the workpiece, in order to bond an optical fiber to the workpiece with an organic epoxy.
Organic epoxies also degrade upon exposure to ultraviolet radiation. In addition, organic epoxies are softened, and may dissolve, by contact with fuels, solvents or oils. Thus, organic epoxies which have degraded or softened may de-bond from the workpiece, such that the optical fiber is no longer bonded to the workpiece and will not detect fractures therein. Furthermore, upon repairing a workpiece, such as an aircraft, the step of stripping paint from the workpiece would also strip or remove the organic epoxy and de-bond the fiber optic from the workpiece.
Accordingly, it is desirable to detect fractures in a metallic workpiece such that the fractures may be repaired before the workpiece suffers additional damage. Some conventional fracture detecting systems, such as illustrated in the '638 patent, stand off or extend outwardly from the surface of the workpiece and significantly alter the profile of the workpiece, such as the profile of an aircraft. In addition, other fracture detecting systems, such as described in the Hale publication, bond optical fibers to a workpiece with a high-modulus adhesive, such as an organic epoxy, which may de-bond in certain conditions such that the optical fibers are no longer affixed to the workpiece and will not detect fractures therein.